The number of people returning to Britain with a potentially fatal form of malaria has risen sharply because travellers are not taking antimalarial drugs, health officials said yesterday.

Cases of falciparum malaria now account for three-quarters of all UK malaria, up from a third of cases two decades ago, according to a Health Protection Agency study.

The majority of malaria occurs among migrants who become infected while visiting west Africa where the disease is endemic. Many do not take antimalarial pills because they believe they are already immune.

"Even though the total number of malaria cases has stayed roughly the same over the years, the proportion that are potentially fatal falciparum malaria has risen steadily," said Peter Chiodini, the director of the HPA's malaria reference laboratory in London.

In a study, published in the British Medical Journal, agency officials examined 39,300 confirmed cases of malaria in Britain between 1987 to 2006. In the first five years 5,120 people contracted falciparum malaria while in the last five years the number of cases had risen to 6,753.

Cases of malaria were concentrated in communities where people frequently travelled to west Africa. Some 96% of falciparum malaria infections followed visits to west Africa, with more than half in travellers returning from Nigeria and Ghana.

Malaria is endemic in at least 150 countries and is caused by a parasite spread by mosquitoes. Of the four forms of the disease, falciparum is the most lethal, accounting for almost all of the three to 16 malaria fatalities in Britain each year.

In the early stages of malaria people show symptoms similar to flu and so may not seek immediate medical help. But caught early enough, malaria is almost always treatable.

The latest findings highlight the widespread failure of health services to alert travellers to the dangers of malaria when they travel abroad. The problem is exacerbated by a common misconception among some migrants that they are resistant to malaria because they lived in a country where the disease is rife. Although people can develop some resistance to malaria it wears off within a year or two of leaving a country.

It is extremely unlikely that mosquitoes in Britain could spread malaria by biting an infected person, although the risk is greater in Italy, Spain and Greece where there are different species of mosquito.

"The key thing is getting the message through to people from the African diaspora living in London and elsewhere, who go back to visit friends and relatives. Many believe malaria is not a big problem for them, but it is. No one can assume they are immune to malaria, even if they've had it repeatedly as a child," said Christopher Whitty, a co-author at the London School of Hygiene and Tropical Medicine.

"The message is not getting through to the people who are most at risk. It's completely preventable and in each of these cases it is potentially fatal," he added.

The agency is now working with migrant communities, principally in the Midlands and London, to raise awareness of the dangers of visiting Africa and other high risk countries without taking antimalarial drugs. Pharmacies may also be trained to offer a range of generic antimalarial drugs, which are cheaper than some brands offered at travel clinics.

"Half of all people who go travelling don't seek any medical advice before going, particularly to tropical destinations where the risk of infectious diseases is significant," said Jane Zuckerman, who runs a travel clinic at the Royal Free and University Medical School in London.

"We have a problem with the general public per se, and then we have a specific problem with ethnic minority groups who perceive they're not at risk from malaria, and how we deal with that is a really major problem."